The XtClass function returns a pointer to the
widget's class structure.

The XtSuperclass function returns a pointer to
the widget's superclass class structure.

The XtIsSubclass function returns
True if the class of the specified widget is equal to or is a subclass
of the specified class. The widget's class can be any number of subclasses
down the chain and need not be an immediate subclass of the specified class.
Composite widgets that need to restrict the class of the items they contain
can use XtIsSubclass to find out if a widget belongs to the desired class
of objects.

The XtCheckSubclass macro determines if the class of the specified
widget is equal to or is a subclass of the specified widget class. The widget
can be any number of subclasses down the chain and need not be an immediate
subclass of the specified widget class. If the specified widget is not a
subclass, XtCheckSubclass constructs an error message from the supplied
message, the widget's actual class, and the expected class and calls XtErrorMsg.
XtCheckSubclass should be used at the entry point of exported routines
to ensure that the client has passed in a valid widget class for the exported
operation.

XtCheckSubclass is only executed when the widget has been compiled
with the compiler symbol DEBUG defined; otherwise, it is defined as the
empty string and generates no code.

To test if a given widget belongs to
a subclass of an Intrinsics-defined class, the Intrinsics defines macros
or functions equivalent to XtIsSubclass for each of the built-in classes.
These procedures are XtIsObject, XtIsRectObj, XtIsWidget, XtIsComposite,
XtIsConstraint, XtIsShell, XtIsOverrideShell, XtIsWMShell, XtIsVendorShell,
XtIsTransientShell, XtIsTopLevelShell, XtIsApplicationShell, and
ZN XtIsSessionShell .